GEOMESA-583 Adding customized encoding/decoding for index entries

* Replacing IndexEntry encode/decode with IndexValueEncoder
* Adding ability to specify attributes to encode in st index value to SimpleFeatureTypes, SftBuilder
* Selecting index iterator in the case of a transform covered by the custom index value
* Improving method profiling
Likewise, do I need stidx=true for the default date?  Is this a/the new standard way to specify which date field goes in the st idx?